The Asian carp's presence is highly contentious in the Midwest, with ramifications that could affect the economy as well as the environment. Here's a primer on the Asian carp and why this invasive species poses such a threat to the Great Lakes ecosystem.

What is the Asian carp?

It's a collective term that describes four species of fish that originated in China but have shown up in the United States: the silver, bighead, grass, and black carp. The bighead and silver carp are the ones that have made their way to the front door of the Great Lakes system.

Grass and black carp can be found farther south, in the Mississippi River. But "they're not knocking at the door [of Lake Michigan] yet," says Jennifer Nalbone, director of navigation and invasive species for Great Lakes United, a coalition of advocacy groups.